This invention relates to scanning radio receivers, and more particularly to scanning radio receivers with frequency synthesizers.
Early scanning receivers used crystals for tuning, and the number of channels available for reception was limited by the number of crystals in a given receiver. A receiver typically contained either eight or sixteen crystals, and different crystals had to be installed to enable reception of different frequencies, including the frequencies of other desired transmissions within range of the receiver as well as, in the case or relocation or portable operation, the assigned frequencies in a different geographical region.
Modern scanning receivers generate local oscillator frequencies with a frequency synthesizer controlled by frequency codes stored in memory. Typically, the memory still contains a small number of frequency codes, but it can be reprogrammed for operation or different frequencies. The channel capacity of scanning receivers has heretofore been limited not only because of the size, cost and complexity of memory circuitry and associated addressing circuitry, but also because of technical limitations on scanning speed. A finite amount of time is required to lock on to each frequency in the active scanning sequence, and then to detect activity on the current channel in order to determine whether or not to continue scanning.
Another limitation of conventional scanning radio receivers involves SEARCH mode operation, in which tuning is performed sequentially in fixed frequency steps. Since the frequency spacings between allocated channels vary from band to band, the least common denominator is commonly used as the frequency step. For example, 5 KHz is a typical frequency step in the aircraft, low VHF and high VHF bands, where channel spacings are 25, 20 and 15 KHz, respectively. A frequency step smaller than conventional channel-to-channel spacing is also provided in some scanners to allow for reception of transmissions on channels between allocated channels. As a result of such scanning techniques, it is common for the frequency step to be less than the receiver bandwidth, whereby scanning can stop, prematurely, on a frequency one or more frequency steps away from the frequency of an active channel. This occurs when the signal on the active channel passes through the IF filters of the receivers with sufficient amplitude to trigger the squelch circuit. Such inaccurate tuning does not occur in SCAN mode, during which the receiver tunes sequentially to user-selected channels, because in SCAN mode the frequency synthesizer is only loaded with frequency codes precisely corresponding to allocated frequencies. However, the small frequency increments encountered in SEARCH mode often leave a conventional scanning receiver susceptible to mistuning and, consequently, poor reception.